The subject matter disclosed herein relates generally to radio-frequency (RF) traps, such as for damping transmission line common-modes in Magnetic Resonance Imaging (MRI) systems.
Electric (E) fields generated by a system can induce currents and voltages that interfere with the operation of the system. For example, MRI systems include a magnet, such as a superconducting magnet that generates a temporally constant (i.e., uniform and static) primary or main magnetic field. MRI data acquisition is accomplished by exciting magnetic moments within the primary magnetic field using radio-frequency RF coils. For example, in order to image a region of interest, the magnetic gradient coils are energized to impose a magnetic gradient to the primary magnetic field. RE transmit coils are then pulsed to create RF magnetic field pulses in a bore of an MRI scanner in order to acquire MR images of the region of interest using, for example, a phased array of RE receiver coils. The resultant image that is generated shows the structure and function of the region of interest.
In MRI systems, the RE coils, besides generating the RF magnetic fields needed for MRI, also generate E fields that induce currents and voltages within transmission lines (e.g., coaxial cables) connected to the one or more RF transmit (e.g., body) or receive coils. These induced voltages and currents, such as common-mode currents can interfere with the operation of the MRI system, such as with the tuning and matching of the RE coils in the MRI system. In order to suppress or dampen these induced modes, one or more cable RF traps, also commonly referred to as balanced-unbalanced networks (baluns), are typically connected in series with cables in the MRI system to provide common-mode energy damping or blocking. In these MRI systems, the cable RF traps are tuned to the system frequency. The blocking impedance at this frequency depends on the inductance and resistance of the trap. Conventional RF traps are usually tuned with a variable capacitor to reach a maximum impedance. However, variable capacitors are costly and typically have a low breaking voltage rating.